1. Field of the Invention
The present invention relates to a semiconductor laser and a method of making a semiconductor laser.
2. Description of the Related Art
A distributed Bragg reflector laser (DBR laser) includes an active layer region and distributed Bragg reflector mirrors (DBR mirrors) provided at both sides of the active layer region. In general, in order to reduce the loss of the DBR mirrors, the energy gap (Eg) of a material constituting the DBR mirrors is set so as to be larger than the Eg of the active layer. Reference 1 (K. Shinoda et al.: The 19th International Conference on Indium Phosphide and Related materials (IPRM 2007), TuB2-3, 39, 15 May 2007) describes a short-cavity DBR laser. This DBR laser includes an active layer region having a multi-quantum well structure composed of InGaAlAs and DBR mirrors in which a diffraction grating is formed on an optical waveguide layer composed of InGaAsP. A butt joint structure is used in order to optically couple the active layer region to the optical waveguide layer including the grating. In the formation of the butt joint structure, a part of an InGaAlAs semiconductor layer grown on the semiconductor substrate is removed by etching, and an InGaAsP semiconductor layer is then regrown. The InGaAsP semiconductor layer constituting the DBR mirrors is composed of a material having a band-gap energy larger than the band-gap energy of the InGaAlAs semiconductor layer constituting the active layer. In addition, a high-reflection (HR) coating film composed of a dielectric film is provided on a back facet which is opposite to a front facet from which a laser beam is emitted.
Japanese Unexamined Patent Application Publication No. 63-255985 discloses a DBR laser. This semiconductor laser is configured so that the band-gap energy of an active layer is controlled to be relatively smaller than the band-gap energy of an optical waveguide region including a diffraction grating by adding an impurity to an active region.